The ores from which uranium is extracted commercially contain only about 0.1% of the element, and the first step, after preconcentration by physical methods, is to leach out the uranium with sulfuric acid or sodium carbonate; it is also possible in some cases to leach the uranium directly from the natural deposit in situ. If sodium carbonate is used, the solution must subsequently be converted to an acid solution. The uranium is then recovered by precipitation with either ammonia or hydrogen peroxide. The ammonia is cheaper, but it precipitates other metallic elements along with the uranium, so that the overall processing, to obtain a suitable pure product, is not necessarily cheaper with ammonia; also, ammonia must be regarded as an environmental pollutant, whereas hydrogen peroxide decomposes to oxygen and water. Both ammonia and hydrogen peroxide are in use commercially. In either case the process of precipitation is a batch operation, and the precipitate is a very fine material which is slow to filter and packs to form a cake.
The solution from which the uranium is precipitated is commonly called mill solution. The present invention relates to an improved method of precipitating uranium peroxide from mill solution in the form of easy-dry, flowable granular particles of significantly increased bulk and flowability. As a result, the common problems of caking, plugging of filters, and the like, usually attendant with commercial ore processing of uranium are avoided.
As earlier indicated, typical uranium processing is usually accomplished by batch process recovery from in situ leach liquors. Batch process recovery is of course less economical than a continuous process might be, and it also seems to lend itself to having excessive amounts of certain impurities present. The impurities that are present often are vanadium and molybdenum, important naturally occurring impurities. These tend to remain in solution when hydrogen peroxide is used to extract uranium peroxide from the mill solution as the precipitant because they are oxidized to soluble perxo complexes VO.sub.5.sup.-3 and MoO.sub.5.sup.-2.
Other problems are attendant when there are high amounts of naturally occurring vanadium in the mill solution. Often the precipitation of the uranium becomes incomplete and the precipitate is contaminated with vanadium. This problem cannot simply be solved by increasing the hydrogen peroxide concentration.
Accordingly, it is a primary objective of the present invention to develop a continuous peroxide precipitation process which produces acceptably pure, fast filtering, easily handled (non-caking) granular form of solid product, uranium peroxide.
Another primary objective of the present invention is to replace the prior art normally used batch processes of uranium recovery from mill solutions with a continuous process which produces easily handleable fast filtering uranium peroxide precipitate.
A yet further objective of the present invention is to develop a process for recovery of uranium from mill solution by the uranium peroxide precipitation technique which can be successfully run, even in the presence of high concentrations of naturally occurring vanadium in the mill solution, even up to the levels of vanadium to uranium of 1:2.
The method and means of accomplishing the above objectives of the present invention will be apparent from the detailed description which will follow hereinafter.